This invention relates generally to thermoplastic melting machines and more specifically to devices for melting and dispensing hot melt adhesive materials.
Thermoplastic adhesive materials are heated and liquified just prior to their application on a substrate and are referred to as "hot melt adhesives". Hot melt adhesives are available in bulk in such forms as chicklets, pellets, slats, bricks and slugs contained in a drum or the like. A drum of bulk adhesive material may be as large as a conventional 55 gallon drum for large scale applications.
The present invention is directed at solving some of the problems encountered with melting adhesive which is initially contained in a slug. Typically, hopper type dispensers have been used to melt a block of adhesives as it contacts a heated melting grid at the bottom of the hopper. A heated reservoir is disposed beneath the melting grid and receives the melted or liquified hot melted adhesive as it passes through the grid. The reservoir maintains the adhesive in a melted liquid state until the application of the adhesive. An outlet of the reservoir typically leads to a pump for pumping the hot melt adhesive to a dispensing apparatus appropriate for the application.
One of the characteristics of many hot melt materials is that they oxidize, char, or degrade when exposed to heat substantially above their melting temperature when exposed to heat in the molten state for a protracted length of time. In the case of many hot melt adhesives, this degradation results in the material loosing its adhesive property and becoming charred. Because of the thermal degradation characteristic, temperature control of the thermoplastic material must be maintained both during melting and for so long as the material is in a molted state. The material must not be exposed to excessive temperatures during melting or until distribution onto a target substrate. One of the factors in determining the time required to melt a given quantity of thermoplastic material is the efficiency of the equipment within which the material is being melted. The evenness of the heat imparted by that equipment to the solid thermoplastic material determines not only the efficiency but the amount of degradation experienced by that solid thermoplastic material during its conversion from the solid to the thermoplastic state. In other words, if the melting apparatus has hot spots, it will subject selected portions of the solid thermoplastic material to excessive temperatures and consequent thermodegradation. On the other hand, if the material has cold spots, it will be relatively inefficient and will take protracted lengths of time to convert a given quantity of solid thermoplastic material from the solid to the molten state.
It is an object of the present invention to provide an improved apparatus for melting thermoplastic materials and for maintaining them in a molten state which provides even heating in a more efficient heating apparatus.
Typical prior art devices for melting solid thermoplastic materials include tank-type melters or grid-type melters. The grid-type melter operates upon the principal of supporting the solid thermoplastic material upon the top of a heated open grid which grid is maintained at a temperature substantially above the melting temperature of the thermoplastic material. The solid material is melted upon contact with this grid and falls through the grid into a reservoir located beneath the grid. From this reservoir, the molten material is pumped to the dispenser. Grid-type melters are generally more efficient than tank-type melters but are generally more expensive because they operate at higher temperatures and subject the molten material to high temperatures and thus requires greater temperature control. Tank-type melters utilize heated walls of a tank to impart sufficient heat to solid thermoplastic material contained within the tank to melt it and convert it to a liquid state. Tank-type melters may utilize a single heating element cast into the bottom wall of the tank to generate the heat required to melt the thermoplastic material. Tank-type melters are thus generally less expensive than grid melters.
It is an object of this invention to provide an improved melter tank for melting and dispensing thermoplastic materials, such as hot melt adhesives, which is more efficient and provides a more even heating than prior tank melters. The improved efficiency derives from the nature and positioning of the heat exchanger units mounted in the tank in which the heat exchanger units are placed above the bottom of the tank and have a predetermined fin design which allows the adhesive to be heated on all sides of the heat exchanger unit and which also allows for better control of the heating elements. The placement of a plurality of heat exchanger units also works to maintain a portion of the solid thermoplastic materials in a molten state covered by the solid thermoplastic materials and to maintain the thermoplastic adhesive material within a close temperature range.